Field of the Invention
The present invention concerns a method to generate an MR image, wherein the MR signals are gradient echoes, and an MR system for implementing such a method.
Description of the Prior Art
In the acquisition of MR images with imaging sequences wherein multiple gradient echoes are read out after the radiation of an RF pulse (known as multiecho sequences), for time reasons and for a better signal-to-noise ratio it is typical to activate what are known as bipolar magnetic field gradients with different polarity. For example, a first gradient echo is switched by a negative magnetic field gradient for dephasing the spins, and the echo is generated by a positive magnetic field gradient. The last portion of the positive magnetic field gradient dephases the excited nuclear spins again, and a magnetic field gradient with opposite polarity (a negative magnetic field gradient in the above example) is switched again to generate an additional echo at a later echo point in time.
The echoes generated with gradients of different polarity are typically designated as even and odd echoes in order to indicate that these echoes have been acquired with bipolar gradients of different polarity. In such readout methods, one challenge is that the even and odd echoes do not both occur exactly in the middle of a readout window, due to the different polarity. This means that the signal depends on the direction in which the associated raw data domain (k-space) is filled with raw data. This slight displacement (shift) in the readout time window leads to a phase shift for the gradient echoes with a first bipolar alignment that differs from the phase shift of the other echo with the opposite bipolar alignment. When a magnitude image is generated from the different MR signals, these different phase shifts play no role. However, if the phase information were to be used, for example for the Dixon technique, for a B0 mapping or for a phase depiction, or for a depiction of the susceptibility, it is difficult to combine these even and odd echoes. In order to be able to completely separately reconstruct the images, both must have complete k-space coverage. In order to reconstruct previous data sets with even and odd echoes, either a complete even data set and a complete odd data set needed to be acquired (which takes longer) or greater artifacts must be accepted given undersampled images.